Actuator

ABSTRACT

An actuator ( 1 ) comprising a motor ( 2 ) assembly, a drive coupling ( 113, 13, 313 ) assembly and an actuator shaft ( 114, 14 ). The motor ( 2 ) assembly comprising a motor housing ( 120, 20 ), having a cover ( 122, 22 ) and a base ( 123, 23 ); an electric motor ( 111, 11, 211, 2 ), comprising an external stator ( 111, 11, 211 ) and an internal rotor ( 112, 12 ); and a hollow output shaft ( 130, 30, 330 ) that is connected co-axially with the internal rotor ( 112, 12 ) such that rotation of the internal rotor ( 112, 12 ) causes a corresponding rotation of the hollow output shaft ( 130, 30, 330 ). The drive coupling ( 113, 13, 313 ) assembly comprises a drive coupling housing ( 115, 15, 315 ) containing a drive coupling ( 113, 13, 313 ), wherein the drive coupling ( 113, 13, 313 ) engages the hollow output shaft ( 130, 30, 330 ) such that rotation of the hollow output shaft ( 130, 30, 330 ) causes a corresponding rotation of the drive coupling ( 113, 13, 313 ). The actuator shaft ( 114, 14 ) extends through the hollow output shaft ( 130, 30, 330 ) and the internal rotor ( 112, 12 ), and engages the drive coupling ( 113, 13, 313 ) such that rotation of the drive coupling ( 113, 13, 313 ), by the hollow output shaft ( 130, 30, 330 ), causes the actuator shaft ( 114, 14 ) to move axially.

FIELD OF INVENTION

The present invention relates to actuators such as valve actuators, andparticularly to electric motor operated valve actuators. The inventionalso relates to systems comprising valves and actuators, and to electricmotors for use in such actuators or systems.

BACKGROUND OF INVENTION

Electric valve actuators are control devices widely used in a largenumber of industries including the process control industry for managingfluid and gas flow. Such actuators use electric motors to transformelectrical energy into mechanical motion to operate fluid controlvalves.

When electric valve actuators are used to operate heavy-duty industrialvalves, a large torque may need to be exerted on the valve. As electricmotors capable of directly providing sufficient torque for suchoperations may be inconveniently large, electric motors with lowertorques can be used with geared drivetrains (often worm gears)configured to increase torque while reducing output speed. The use ofsuch gears to transmit power from the electric motor to the valveincreases mechanical complexity as more moving parts are involved.Geared electric valve actuators can therefore suffer from reducedefficiency from friction losses, reduced precision from gear backlash,wear/lubrication/maintenance issues and increased noise. The drivetrainalso occupies space in the actuator housing leading to a larger overallsize.

There is therefore a need for a valve actuator comprising an electricmotor capable of compact assembly within the valve actuator housing andcapable of providing sufficient torque to operate heavy-duty industrialvalves without a large geared drivetrain.

EP 3026373 A1 (TGK CO), EP 0364308 A2 (SNEDDON) and GB 862324 A (ATOMICENERGY AUTHORITY UK) disclose motor operated valve actuators.

SUMMARY OF INVENTION

One aspect of the invention provides an actuator comprising a motorassembly, a drive coupling assembly and an actuator shaft. The motorassembly comprising a motor housing, having a cover and a base; anelectric motor, within the motor housing, comprising an external statorand an internal rotor; and a hollow output shaft that is connectedcoaxially with the internal rotor such that rotation of the internalrotor causes a corresponding rotation of the hollow output shaft. Thedrive coupling assembly comprises a drive coupling housing containing adrive coupling, wherein the drive coupling engages the hollow outputshaft such that rotation of the hollow output shaft causes acorresponding rotation of the drive coupling; and wherein the drivecoupling assembly is connected to the base of the motor housing. Theactuator shaft engages the drive coupling such that rotation of thedrive coupling, by the hollow output shaft, causes the actuator shaft tomove.

The drive coupling assembly can be external to the motor housing. Forexample, the electric motor can be on one side of the base of the motorhousing (on the internal side of the motor housing) and the drivecoupling assembly can be on the other side of the base of the motorhousing (external to the motor housing).

The connection between the drive coupling assembly and the motor housingcan be releasable. For example, the drive coupling assembly and themotor housing can be connected to each other via at least one removablefixing element (e.g. a bolt).

The hollow output shaft can directly engage the drive coupling or canengage the drive coupling through an arrangement of gears. Thearrangement of gears can comprise a gearbox, and the drive couplingassembly can be connected to the base of the motor housing by means ofthe gearbox. The gearbox can contain a planetary gear arrangementincluding a sun gear and planet gears mounted on a rotary gear carrier,wherein the sun gear is hollow and coaxial with the hollow output shaftand the actuator shaft, and rotation of the hollow output shaft can betransmitted to the actuator shaft by rotation of the rotary gearcarrier. The planetary gear arrangement can be a multi-stage planetarygear arrangement.

The drive coupling can comprise an anti-back drive coupling.

The actuator shaft can extend through the hollow output shaft and theinternal rotor. In this case, rotation of the drive coupling can causethe actuator shaft to move axially.

The rotation of the internal rotor can be transmitted to the drivecoupling via engagement of an at least one axially-extending lug and anat least one corresponding cut-out formed in respective parts of thehollow output shaft and drive coupling. This engagement can allow themotor assembly to be easily moved away from the drive coupling assemblyand the actuator shaft (e.g. for maintenance), as the lug(s) andcut-out(s) can be disengaged by simply moving the motor assembly axiallyaway from the drive coupling assembly once the connection between thedrive coupling assembly and the motor housing has been released.Moreover, this can be done without affecting the position of theactuator shaft as the connection between the actuator shaft and themotor assembly is only indirectly via the drive coupling assembly.

The hollow output shaft can be mounted within the internal rotor andextend through the internal rotor.

The cover and base of the motor housing can define an internal tube andthe internal rotor can be mounted on bearings on the tube.

The cover and base of the motor housing can be secured in a spacedarrangement by locating members extending between peripheral regions ofthe cover and base of the motor housing. In this case, bearings can beprovided in the cover and base of the motor housing to mount theinternal rotor for rotation. Additionally, the hollow output shaft canhave an external flange at its upper region, and the hollow output shaftcan be mounted within the internal rotor by means of the flange, and anupper surface of the flange engages the bearing in the upper portion ofthe housing.

The cover of the motor housing can extend from the base of the motorhousing around the stator and the rotor.

The cover of the motor housing can have open regions, the motor housingfurther comprising a removable lid that can be secured to the externalsurface of cover of the motor housing to close the open regions.

The actuator can comprise an actuator housing within which actuatorcomponents are secured and defining the motor housing onto which theexternal stator and internal rotor are directly mounted. Alternatively,the actuator housing can define the drive coupling housing, and forexample, the actuator housing can enclose the motor assembly.

The actuator can comprise a valve housing, within which valve componentsare secured, that defines the drive coupling housing.

The drive coupling can be an internally threaded drive nut, a ball screwnut or a roller screw nut that engages a thread on the exterior of theactuator shaft. The drive coupling can comprise an anti-back drivecoupling

The external stator, internal rotor, hollow output shaft, drive couplingand actuator shaft can be arranged coaxially. For example, internalrotor, hollow output shaft, and the drive coupling can have a commonaxis of rotation.

The external stator can comprise a stator body having a series ofinwardly projecting teeth, a winding being provided around each tooth.In this case, the sides of the stator teeth can be substantiallystraight along the whole length of the tooth. Electric motors havingthis feature can also be used in other applications.

The external stator can be held in position by engagement with the coverand base of the motor housing.

The drive coupling housing can be fixed relative to a lower surface ofthe lower portion. The actuator can further comprise a bearing assemblybetween the drive coupling and the drive coupling housing such that thedrive coupling is allowed to rotate relative the drive coupling housing.

Another aspect of the invention provides a system comprising an actuatoraccording to the first aspect, and a valve connected to the actuator.

The actuator shaft can be directly connected to the valve.

A further aspect of the invention provides a system comprising anactuator according to the first aspect, wherein the actuator shaft is athreaded valve stem comprising a valve at its lower end.

A further aspect of the invention provides an electric motor for use inan actuator or a system as defined above, comprising an external statorand an internal rotor, wherein the external stator comprises a statorbody having a series of inwardly projecting teeth, a winding beingprovided around each tooth; and wherein the sides of the stator teethare substantially straight along the whole length of the tooth.

Further variants are disclosed in the following description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a part section view of an electric valve actuator.

FIG. 2 illustrates an exploded view of an actuation system.

FIG. 3 illustrates a cross section view of the assembled actuationsystem shown in FIG. 2.

FIG. 4 illustrates a cross section view of a second actuation system.

FIG. 5 illustrates a cross section perspective view of the secondactuation system at an angle.

FIG. 6 illustrates the exterior of the second actuation system.

FIG. 7 illustrates a cross section view of the second actuation systemin a partially-disassembled configuration.

FIG. 8 illustrates the exterior of the second actuation system in thepartially-disassembled configuration.

FIG. 9 illustrates a further valve actuation system including aplanetary gear arrangement.

FIG. 10 illustrates a cross section view of an electric motor designedfor use with a actuation system.

DETAILED DESCRIPTION

FIG. 1 shows a part section view of an electric valve actuator 1,comprising an electric motor 2 (for example, a brushless DC electricmotor or an AC induction motor) for electric powered operation of avalve (not shown); a drivetrain 3 that connects the motor 2 to the valve(for example, via a worm gearing); a terminal compartment 4 forelectrical power and data signal connections; an actuator control unit 5for processing command signals and actuator signals for controllingactuator operations; a position control unit 6 connected to thedrivetrain; and a display 7 and local controls 8 for locally viewing andconfiguring the operation of the actuator 1. A hand wheel 8 for manualoperation of the valve in emergency situations (e.g. when the electricmotor 2 is malfunctioning or when no mains power is available) can alsobe provided.

FIG. 2 illustrates an exploded view of an actuation system 10 that canbe used in place of the electric motor 2 and drivetrain 3 describedabove in relation to FIG. 1. The actuation system 10 can be used toprovide a compact electric valve actuator capable of providingsufficient torque to operate heavy-duty industrial valves without theneed for a large drivetrain/gears. FIG. 2 is a simplified illustrationof the actuation system 10 with components such as electronic devices,PCBs and wires omitted for clarity.

The actuation system 10 comprises an electric motor, such as a brushlessDC motor or a switched reluctance motor, comprising an external stator11 and an internal rotor 12; a motor housing 20; a hollow output shaft30; a drive coupling 13 contained within a drive coupling housing 15;and an actuator shaft 14. As shown in FIG. 3, all of these componentsare arranged on a common central axis which is also the axis of rotationfor the internal rotor 12, hollow output shaft 30 and drive coupling 13.

The internal rotor 12 is hollow, having a central, axially-extendingbore. As shown in FIG. 3, the internal rotor 12 is substantiallycontained within the external stator 11 such that the inner surface ofthe stator 11 surrounds the outer surface of the internal rotor 12.

Both the external stator 11 and internal rotor 12 are mounted within agenerally circular motor housing 20 by which the system 10 can belocated in an actuator housing. The motor housing 20 comprises a base23, a cover 22, and a lid 21, each having a central aperture. The drivecoupling housing 15 is located on a lower surface of the base 23 (i.e.the external surface of the base 23/the surface of the base 23 externalto the motor enclosing space of the motor housing 20). The cover 22 isheld in a spaced arrangement with the base 23 by means of locatingmembers 25 between peripheral regions of the cover 22 and base 23 so asto secure the stator 11 and rotor 12 in place within the motor housing20. The cover 22 has open regions 26, and the lid 21 is removably fixedto the upper surface of the cover 22 (i.e. the external surface of thecover 22/the surface of the cover 22 external to the motor enclosingspace of the motor housing 20) so as to close the open regions 26.

The locating members 25 can comprise threaded rods and nuts, thethreaded rods extending through clearance counterbore holes in the cover22 and base 23 of the motor housing.

The lid 21 and/or cover 22 of the motor housing 20 can provide means ofattaching components (such as electronic components and cables) to themotor housing 20.

Bearings 27, 28 are provided in the cover 22 and base 23 of the motorhousing 20 to allow the internal rotor 12 to be rotatable in the motorhousing 20.

Alternatively, the electric motor 11, 12 can be mounted directly ontothe actuator housing, instead of being mounted within the abovedescribed motor housing 20, in which case bearings would be provided inthe actuator housing to allow the internal rotor 12 to be rotatable inthe actuator housing. Moreover, the stator 11 would be fixed to theactuator housing by means such as heat shrinking, bonding or mechanicalkey fitting(s) to prevent rotation of the stator 11 within the actuatorhousing.

The hollow output shaft 30 comprises a hollow cylinder having anexternal flange 31 protruding radially from the outer surface at itsupper end region. The hollow output shaft 30 is mounted within theinternal rotor 12 by means of fixing means, such as bolts, between theflange 31 and the internal rotor 12 to allow the hollow output shaft 30to rotate with the internal rotor 12. The upper surface of the flange 31engages the bearing 27 in the upper portion of the housing (e.g. upperportion 22 or an upper portion of the actuator housing). The lower endof the rotor 12 engages the bearing 28 in the lower portion of thehousing (e.g. base 23 or a lower portion of the actuator housing). Thelower end of the shaft 30 projects through the central aperture of thelower portion of the housing (e.g. base 23 or a lower portion of theactuator housing).

The drive coupling housing 15 is fixed to the lower portion of the motorhousing 20 (i.e. base 23) or actuator housing around the centralaperture. The drive coupling 13 is mounted for rotation in bearings 16in the drive coupling housing 15. The hollow output shaft 30 hasaxially-extending lugs 32 a, 32 b at one end (i.e. its lower end). Thelugs 32 a, 32 b engage in corresponding cut-outs 33 a, 33 b provided atone end of the drive coupling 13 such that the drive coupling 13 canrotate in the bearings 16 with the hollow output shaft 30.Alternatively, axially-extending lugs can be provided at one end of thedrive coupling 13 and corresponding cut-outs can be provided at one endof the hollow output shaft 30. As such, rotation of the internal rotor12 is transmitted to the drive coupling 13 via engagement of the lug(s)and cut-out(s) formed in respective parts of the hollow output shaft 30and the drive coupling 13.

The drive coupling 13 comprises a drive nut having an internallythreaded bore. The valve actuator shaft 14 extends through the hollowoutput shaft 30 and drive coupling 13 and has an external thread thatengages the internal thread of the drive coupling 13. This engagementallows rotation of the drive coupling 13 relative to the actuator shaft14 to be converted into linear movement of the shaft 14. Thus, actuationof the rotor 12 moves the actuator shaft 14 axially. If a valve isprovided at one end (i.e. the lower end) of the actuator shaft 14 (e.g.wherein the actuator shaft is directly connected to the valve, orwherein the actuator shaft is a threaded valve stem comprising a valveat its lower end), the axial movement of the actuator shaft 14 in turnoperates the valve allowing the electric motor 11, 12 to directly act onthe valve. The pitch of the threads on the drive coupling and shaft 14determine the degree of axial movement of the shaft per turn of therotor 12.

As an alternative to the drive nut described above, the drive coupling13 can be a recirculating ball screw nut or a roller screw nut. Also,the actuation system 10 can comprise an anti-back drive coupling.

As with the actuator of FIG. 1, a hand wheel (not shown) can be providedfor manual operation of the valve in emergency situations (e.g. when theelectric motor is malfunctioning or when no mains power is available).

FIGS. 4-8 illustrate a second, alternative actuation system 100 that canalso be used in place of the system described above in relation to FIGS.2-3. FIGS. 4-8 are a simplified illustration of the second actuationsystem 100 with components such as electronic devices, PCBs and wiresomitted for clarity.

The second actuation system 100 comprises an electric motor comprisingan external stator 111 and an internal rotor 112; a motor housing 120; ahollow output shaft 130; a drive coupling 113 contained within a drivecoupling housing 115; and an actuator shaft 114, all of which aregenerally configured in the same manner as the system shown in FIGS.2-3.

In the second system, the cover 122 extends from the base 123 of themotor housing around the stator 111 and rotor 112, defining an internaltube 122 a, so as to secure the stator 111 and rotor 112 in place withinthe motor housing 120. The internal rotor 112 is mounted on bearings127, 128 on the internal tube 122 a to allow the internal rotor 112 tobe rotatable within the motor housing 120. The internal rotor 112 isalso mounted on a seal 124 on the internal tube 122 a and a seal 129 inthe central aperture of the lower portion of the housing (e.g. base 123or a lower portion of the actuator housing) to allow components of theelectric motor 111, 112 (e.g. stator windings of the electric motor,motor drive electronics and IGBT modules) to be sealed, for example,against potentially explosive atmospheres. Moreover, the cover 122 canhave open regions, and the motor housing 120 can further comprise a lidthat can be removably fixed to the upper surface of the cover 122 (i.e.the external surface of the cover 122/the surface of the cover 122external to the motor enclosing space of the motor housing 120) so as toclose the open regions 26. At least one seal can be provided between thelid and the open regions.

The cover 122 and/or lid of the motor housing 120 can provide means ofattaching components (such as electronic components and cables) to themotor housing 120.

Similar to the system shown in FIGS. 2-3, the hollow output shaft 130has axially -extending lugs 132 a, 132 b at one end (i.e. its lowerend/the end that extends through the central aperture of the lowerportion of the housing (e.g. base 123 or a lower portion of the actuatorhousing)) that engage in corresponding cut-outs 133 a, 133 b (see FIG.8) provided at one end of the drive coupling

In the systems described above, drive coupling assembly is connected tothe base of the motor housing. The drive coupling assembly is alsoexternal to the motor housing as the electric motor is on one side ofthe base of the motor housing (i.e. on the internal side of the motorhousing) and the drive coupling assembly is on the other side of thebase of the motor housing (i.e. on the external side to the motorhousing). The connection between the drive coupling assembly and themotor housing is releasable as the drive coupling assembly and the motorhousing are connected to each other via at least one removable fixingelement. For example, in compact actuation systems 10 and 100 theremovable fixing elements comprise bolts 41, 141 that extend throughclearance holes provided on a flange 42, 142 of the drive couplinghousing 15, 115 and engage threaded holes in the base 23, 123 of themotor housing.

The engagement between the lug(s) and the corresponding cut-out(s)formed in respective parts of the drive coupling (i.e. drive coupling 13or drive coupling 113) and the hollow output shaft (i.e. shaft 30 orshaft 130) allows the motor assembly to be easily moved away from thedrive coupling assembly and the actuator shaft (e.g. for maintenance),as shown in FIGS. 7 and 8, as the lug(s) and cut-out(s) can bedisengaged by simply moving the motor assembly axially away from thedrive coupling assembly once the connection between the drive couplingassembly and the motor housing has been released. Moreover, this can bedone without affecting the position of the actuator shaft (i.e. shaft 14or shaft 114) as the actuator shaft is indirectly connected to the motorassembly via the drive coupling assembly (i.e. the engagement betweenthe lug(s) and cut-out(s)). If a valve is provided at one end of theactuator shaft (i.e. the lower end of the actuator shaft), this meansthe motor assembly can be safely removed for repair or replacementwithout taking the valve out of service.

The drive coupling housing (i.e. drive coupling housing 15 or drivecoupling housing 115) can be connected to a valve housing, within whichthe valve components are secured. Alternatively, the valve housing candefine the drive coupling housing (i.e. drive coupling housing 15 ordrive coupling housing 115).

Where the electric motor to be used with the compact actuation systems10 and 100 is a brushless DC motor, drive electronics and IGBT(Insulated-Gate Bipolar Transistor) modules can be used to switch thecurrent to the motor windings using pulse-width modulation (PWM)control. The IGBT modules can be arranged radially around the motor endwindings and electrically connected to the motor end windingterminations.

Epoxylite® (or a similar material known in the art) can be used tosubstantially or completely encapsulate components of the electric motorto be used with the compact actuation system 10 and 100 (e.g. the statorwindings of the electric motor, motor drive electronics and IGBT modulesat the end of the motor end windings). For example, this can be achievedby using the motor housing 120 as a mould and pouring molten Epoxylite®(or a similar material) into an opening of the motor housing 120 (e.g.an opening provided between the internal rotor 112 and the base of themotor housing 123) and allowing the Epoxylite® (or a similar material)to solidify. Encapsulating the components of the electric motor inEpoxylite® (or a similar material), can allow the compact actuationsystem (i.e. compact actuation system 10 or 100) to be explosion proof,as the Epoxylite® (or a similar material) acts as a barrier betweenpotentially explosive atmospheres and the encapsulated components thatcan be sources of spark ignition (such as the motor windings and driveelectronics).

In the embodiments show in FIGS. 2-8, the hollow output shaft 30, 130directly engages the drive coupling 13, 113. In such cases, the maximumtorque that can be applied to the actuator shaft will depend on the sizeof the motor. For high torque applications, the size of motor necessaryfor direct drive may be too large to be practical. In this case, a geararrangement can be interposed between the motor and the drive coupling.FIG. 9 illustrates an embodiment of this arrangement, which includes agearbox 350 housing a planetary gear arrangement including a sun gear352 and a series of planet gears 354. The sun gear 352 is hollow and ismounted concentrically with the hollow output shaft 330 and drivecoupling 313 in the drive coupling housing 315 (and hence with theactuator shaft, not shown). The sun gear 352 is mounted on a sun gearcarrier 356 which is mounted in bearing in the upper part of the gearbox350 and has cut-outs 333 to receive lugs 332 on the end of the hollowoutput shaft 330. The planet gears 354 are mounted on a planet gearcarrier 358 which rotates with the planet gears 354 and transmits therotation to the drive coupling 313 through drive lugs 360. The motor andgearbox can be separate units so that the motor can be removed from thegearbox. Similarly, the gearbox can be removed from the drive couplingand so leave the valve in position, as is discussed above.

While FIG. 9 shows a single stage planetary gear system, it is alsopossible to use a multi-stage planetary gear system. For example, theplanet gear carrier can be connected to a hollow sun gear of asubsequent planetary gear stage. Two or more subsequent stages can beused.

As an alternative to the planetary gearbox disclosed above, the gearboxcould be spur gear type or bevel gear type with a hollow output shaft toallow the actuator shaft to rise up through the gearbox output gear.Where such a spur gear or bevel gear gearbox is used then the actuatorshaft may not rise up through the hollow output shaft of the motor asthe motor rotation axis may no longer be concentric with the gearboxoutput axis.

The direct acting motor arrangement described above, or the directacting motor coupled to a gearbox could be used in conjunction with ananti-back drive coupling to operate a ¼ turn valve such as a ball valveor butterfly valve. This type of ¼ turn valve does not have a risingvalve stem or actuator shaft. The anti-back drive coupling couldpotentially be left attached to the valve to maintain the position ofthe valve whilst the motor was removed for maintenance or replacement ina similar way to what is described above.

FIG. 10 shows a cross section view of an electric motor 211, 212 thatcan be used with the compact actuation systems 10 and 100 describedabove. The stator 211 consists of an external stator body 213 with aseries of inwardly projecting stator teeth 214. The sides of the statorteeth 214 are substantially straight along the whole length of eachtooth. It has previously been proposed to provide lateral extensions ortips at the end of each tooth (i.e. the sides extend laterally towardsthe adjacent tooth at the end). This has been done to reduce high levelsof flux density in the stator leading to excess heat being generated inuse. In effect, the external stator 211 has no stator tooth tips. Inapplications such as the valve actuator described above, the motor canhave low rotational output speeds such that iron losses and heatgeneration are less of an issue. Also, the motor is to be operated, forexample, with a 25% duty cycles rather than being rated to runcontinuously and the external stator 211 can be relatively large insize, relative to the motor output power, and thus has a large thermalmass that helps distribute any heat generated.

Lack of stator tooth tips on the external stator 211 can substantiallysimplify the coil assembly process as pre-wound concentrated ‘coilpacks’ can be assembled onto the stator 211 by simply sliding them ontothe stator teeth, as opposed to having to wind coils onto the stator 211in situ. The process of carrying out maintenance on such a stator 211 isalso simplified as said ‘coil packs’ can be easily removed from thestator teeth by simply sliding them away from the stator teeth.Moreover, it simplifies the design of the external stator 211 making iteasier to manufacture. A motor having these features, such as abrushless DC motor or a switched reluctance motor, can find use inapplications in addition to the valve actuator disclosed above. Thisinvention extends to such uses.

Further changes can be made within the scope of this invention.

1. An actuator, comprising: a motor assembly, comprising: a motorhousing, comprising: a cover, and a base; an electric motor, within themotor housing, comprising: an external stator, and an internal rotor;and a hollow output shaft that is connected coaxially with the internalrotor such that rotation of the internal rotor causes a correspondingrotation of the hollow output shaft; a drive coupling assembly connectedto the base of the motor housing, the drive coupling assembly,comprising a drive coupling housing containing a drive coupling, whereinthe drive coupling engages the hollow output shaft such that rotation ofthe hollow output shaft causes a corresponding rotation of the drivecoupling; and an actuator shaft that engages the drive coupling suchthat rotation of the drive coupling, by the hollow output shaft, causesthe actuator shaft to move.
 2. The actuator as claimed in claim 1,wherein the drive coupling assembly is external to the motor housing. 3.The actuator as claimed in claim 2, wherein the connection between thedrive coupling assembly and the motor housing is releasable so that themotor housing can be removed from and reattached to the drive couplingassembly
 4. The actuator as claimed in claim 1, wherein the hollowoutput shaft directly engages the drive coupling.
 5. The actuator asclaimed in claim 1, wherein the hollow output shaft engages the drivecoupling through an arrangement of gears.
 6. The actuator as claimed inclaim 5, wherein the arrangement of gears comprises a gearbox, and thedrive coupling assembly is connected to the base of the motor housing bymeans of the gearbox.
 7. The actuator as claimed in claim 6, wherein thegearbox contains a planetary gear arrangement including a sun gear andplanet gears mounted on a rotary gear carrier, wherein the sun gear ishollow and coaxial with the hollow output shaft and the actuator shaft,and rotation of the hollow output shaft is transmitted to the actuatorshaft by rotation of the rotary gear carrier.
 8. The actuator as claimedin claim 7, wherein the planetary gear arrangement is a multi-stageplanetary gear arrangement.
 9. The actuator as claimed in claim 6,wherein the drive coupling comprises an anti-back drive coupling. 10.The actuator as claimed in claim 1, wherein the actuator shaft extendsthrough the hollow output shaft and the internal rotor.
 11. The actuatoras claimed in claim 10, wherein rotation of the drive coupling causesthe actuator shaft to move axially.
 12. The actuator as claimed in claim1, wherein rotation of the internal rotor is transmitted to the drivecoupling via engagement of an at least one axially extending lug and anat least one corresponding cut-out formed in respective parts of thehollow output shaft and drive coupling.
 13. The actuator as claimed inclaim 1, wherein the cover of the motor housing extends from the base ofthe motor housing around the stator and rotor.
 14. The actuator asclaimed in claim 13, wherein the cover of the motor housing defines aninternal tube and the internal rotor is mounted on bearings on the tube.15. The actuator as claimed in claim 1, wherein the cover and base ofthe motor housing are secured in a spaced arrangement by locatingmembers extending between peripheral regions of the cover and base ofthe motor housing.
 16. The actuator as claimed in claim 15, whereinbearings are provided in the cover and base of the motor housing tomount the internal rotor for rotation.
 17. The actuator as claimed inclaim 16, wherein the hollow output shaft has an external flange at itsupper region, and the hollow output shaft is mounted within the internalrotor by means of the flange, and an upper surface of the flange engagesthe bearing in the upper portion of the housing.
 18. The actuator asclaimed in claim 15, wherein the hollow output shaft is mounted withinthe internal rotor and extends through the internal rotor.
 19. Theactuator as claimed in claim 1, wherein the cover of the motor housinghas open regions, the motor housing further comprising a removable lidthat can be secured to the external surface of the cover of the motorhousing to close the open regions.
 20. A system comprising: an actuatorcomprising: a motor assembly, comprising: a motor housing, comprising: acover, and a base; an electric motor, within the motor housing,comprising: an external stator, and an internal rotor; and a hollowoutput shaft that is connected coaxially with the internal rotor suchthat rotation of the internal rotor causes a corresponding rotation ofthe hollow output shaft; a drive coupling assembly connected to the baseof the motor housing, the drive coupling assembly, comprising a drivecoupling housing containing a drive coupling, wherein the drive couplingengages the hollow output shaft such that rotation of the hollow outputshaft causes a corresponding rotation of the drive coupling; and anactuator shaft that engages the drive coupling such that rotation of thedrive coupling, by the hollow output shaft, causes the actuator shaft tomove, and a valve connected to the actuator.
 21. The system as claimedin claim 20, wherein the actuator shaft is directly connected to thevalve.
 22. The actuator as claimed in claim 1, wherein the externalstator comprises a stator body having a series of inwardly projectingteeth, a winding being provided around each tooth.
 23. The actuator asclaimed in claim 22, wherein the sides of the stator teeth aresubstantially straight along the whole length of the tooth.
 24. Anelectric motor for use in an actuator as claimed in claim 1, having anexternal stator and an internal rotor, wherein the external statorcomprises a stator body having a series of inwardly projecting teeth, awinding being provided around each tooth; and wherein the sides of thestator teeth are substantially straight along the whole length of thetooth.